Marner is more exciting and explosive. I get more of a "Wow" factor watching Mitch and when he's on top of his game and puts it all together, he's dynamic and dangerous. He has the ability to put guys back on their heels more often. He also is better with more space, which is why he's more dangerous on the PP, where there is more time and space.
Aho isn't as dynamic, but like you said, he's looks effortless and is more fluid. He's got a better shot, is a more natural goal scorer, has deceptive speed, and he's stronger than Mitch. Both are good defensively and will only get better.
The people that say one has better vision or hockey sense than the other (which I know isn't directly measurable), just don't watch the two or are way too biased. Aho is one of the most instinctive hockey players I've ever seen. His movement without the puck is uncanny and his vision is incredible. There's a reason people in Finland call him the professor because he sees the ice so well, with or without the puck. Marner, similarly has fantastic vision as well and his ability to see the ice at speed is incredible. Both of them are incredibly smart and have superb vision.
I think Marner probably has more upside offensively if he puts it all together.
Good take, I watch the Canes a fair amount as well (not because they are my 2nd team, but they are one of the East teams I keep an eye out on, with the other Canadian teams + Ducks, as I have the NHL package)
I'd add that both players don't float as much as other "young stars" I refuse to name. They are limited by their system, and coaches to some extent (Peters being a Babcock protege).
Marner is often the short-pass option to exit the zone, Aho in a similar but lesser vein (he back-checks more than his young peers that put up higher totals). Other young stars just leak and float out regularly, with much less defensively responsibility. It's like Kessel and his productivity in the run and gun, Wilson system.
I think if both players were in a more open-ice system, they'd rack up more points, but that wouldn't make them better or more responsible players.....nor is that playstyle sustainable if your trying to win. Though admittedly, I think Marner's had more growing pains this year (line shuffling, being forced to work on his cycle/loose puck battle game, so points aren't a great determinant for how his season has gone, it's very, very much been the definition of a developmental year for him.
Lastly, Aho could have better linemates. And Marner certainly has had better linemates for the last few weeks. But JVR and Bozak at even strength are awful and have limited the productivity of Marner for at least a year and half now. I'm not complaining (hopefully that brings down Marner's hit + term), but it needs to be said. It goes beyond "precedent", JVR and Bozak are declining players in the evolving league, that is predicated heavily on
A) play with speed through the neutral zone
B) older school teams that have "heavy" players that impose their will on the cycle
JVR, and Bozak are poor at A) and B) and quite one-dimensional. Needless to say, Marner would work better with a heavier player that could finish well (he can account for A) as well as most players in the league).